The present invention relates generally to apparatus and methods for in situ construction of subsurface containment barriers for containing hazardous waste materials buried under the earth, and more particularly to a method of constructing a vault to encapsulate such hazardous materials so that contaminants are not released into the air or surrounding or underlying strata. The present invention further relates to a means for monitoring the continued integrity of the vault over many years and to a means for repairing any breaches which might occur over time.
In the early days of the nuclear age, contaminated debris and undocumented low level radioactive waste were buried in shallow trenches. Other waste materials were placed in underground storage tanks. These burial areas are now considered to pose a unacceptable risk to the environment. Excavation and removal of these wastes is potentially dangerous and very expensive. The concern is that excavation of such sites could release airborne radioactive contaminants which would pose a substantial harm to personnel and nearby residents. There have been a number of solutions proposed for containing these sites. Some of these solutions include slant drilled jet grouting, soil freezing, soil dehydration, tunneling, and chemical grout permeation. Others have taught vertical drilling and hydraulic fracturing as a means of forming a bottom barrier.
U.S. Pat. No. 4,230,368 and 4,491,369 to Cleary and others have disclosed the concept of displacing soil blocks containing the contaminants. This is accomplished by making a narrow vertical trench around the perimeter of the soil and forming a horizontal fracture under the site, through injection of a fluid under pressure. The horizontal fracture intersects the vertical perimeter trench. A seal is created along the surface areas of the vertical perimeter trench as continued injection of pressurized fluid into the horizontal fracture causes the block of soil within the perimeter to be lifted upwards.
The injected fluid may also become a sealant to produce a barrier surrounding the block like a basement. U.S. Pat. No. 4,230,368 to Cleary discloses that the density of the fluid is a factor in reducing the pressure needed to displace the block but does not contemplate use of fluid densities greater than those achievable with locally excavated soil materials in a clay slurry. This is by definition, less dense than soil. Gel strength of the fluid is mentioned as the primary means of sealing the perimeter opening. Such methods produce both the initial fracture and upward displacement by increasing hydrostatic pressure on the bottom of the block.
The problem with this approach is that hydrostatic pressure will cause fractures to propagate along the plane of least principal stresses. It is not possible to verify the final location and limits of such fractures in a radioactive waste site. The thickness and continuity of such fractures can not be verified. Because of the potential for uncontrolled fracturing into and beyond the contaminated material this method has not been used to produce any type of containment structure in radioactive waste sites.
The inventor""s previous invention, U.S. Pat. No. 5,542,782, which is hereby incorporated by reference, describes a means of cutting vertical and horizontal barriers with high pressure jets of grout slurry and teaches the benefits of constructing such barriers from grout materials which are of a density equal to or greater than that of the overburden. This reference also teaches that the thickness of a horizontal grout barrier may be increased by introduction of a grout slurry which is sufficiently dense so as to result in net upward forces on the soil which heave the land surface upward, however few details of the method or apparatus to accomplish this are described.
The present invention is directed to improved methods and apparatus for constructing a thick horizontal barrier through buoyant block displacement. The present invention provides a new means for cutting the soil with a cable saw and details a practical apparatus for introducing a block displacement fluid to multiple cuts under a large multi-acre site. The subject invention also provides an improved means of cutting a thin horizontal barrier with high pressure jetting apparatus, which is more practical for application of chemical grouts and has an improved means of joining adjacent cuts to previous ones and recovering from equipment breakage.
The present invention uses a combination of trenching, horizontal directional drilling, diamond wire quarry saw methods, or high pressure jetting to cut a thin gap under and around a block of soil containing the contamination. As this xe2x80x9ccutxe2x80x9d is formed, it is filled with a high-density low-viscosity fluid grout. This thin channel of this dense fluid extends back to the surface and so exerts a hydrostatic head against the soil. This proprietary fluid is so heavy that the soil and rock will literally float on a thin layer of the fluid. This keeps the cut open and prevents the weight of the soil block from squeezing the fluid out from under it. After the block has been completely cut loose from the earth, additional dense fluid is pumped and poured into the cut. This additional fluid exerts a buoyant force on the block and causes it to rise out of the earth. The dense fluid is designed to slowly harden over a period of weeks to form an impermeable barrier. Use of the head of the dense grout fluid instead of attempting to pressurize the fluid to support the block is a subtle but important innovation. It eliminates the difficulties of sealing the vertical perimeter trench and also prevents uncontrolled fracturing of the grout into the waste burial area. If any of the grout fluid should find a crack in the active waste area it will do no more than fill it. It can not spurt up to the surface and form fountains of contaminated liquid, as it could do if it were under pressure. While the grout under the block is liquid an impermeable barrier sheet, such as HDPE (high density polyethylene extrusion), may be pulled under the floating block.
After the xe2x80x9cmoat-likexe2x80x9d barrier around the soil block has hardened, a gravity-anchored, airtight cap structure is built on top of it. The HDPE liner under the block may be fission bonded to the HDPE liner in the cap to achieve a very high degree of containment integrity. Passive soil gas pressure sensors under the cap and similar sensors in the ground outside the cap monitor the air pressure changes inside the structure as a function of normal atmospheric pressure changes due to weather. This data allows passive monitoring of the integrity of not only the horizontal barrier but also the entire containment structure. Moisture, sound, and chemical tracer levels may be passively monitored as leak and leak location indicators, Repair of damage is also possible by flooding the structure with liquid grout.
A wire saw may also be used with molten paraffin grout to form a thin barrier roughly the thickness of the steel cable. This method maintains a circulating supply of molten paraffin in the pulling pipes which is ejected through holes in the pipe adjacent to the area being cut. The steel cable carries this molten paraffin into the cut and back to the surface. The paraffin is modified with additives that cause it to permeate into tight soils and form a barrier significantly thicker than the cut. Rapid cooling of the grout as the cut proceeds prevent excessive subsidence. An unlimited number of replacement jetting tubes or wire saw cables may be pulled into cutting position by the steel cables or the heated xe2x80x9cpulling pipesxe2x80x9d which are in the original directionally drilled holes. These may remelt a path through the previous cut.
Improvements on the inventor""s previously disclosed method of forming a barrier by high pressure jetting from a long arcuate conduit are also described. The new method forms a very thin cut using chemical grout, such as molten paraffin or molten low density polyethylene, circulated through an catenary arcuate tube at high pressure and rate while the tube itself is reciprocated through directionally drilled holes to the advancing cut. Holes or hardened ports in the forward facing surface of the tube eject the heated liquid into the soil at high kinetic energy causing the soil to be eroded and substantially replaced by the molten paraffin. The tube is also able to perform abrasive cutting. An unlimited number of replacement jetting tubes or wire saw cables may be pulled into cutting position by the heated xe2x80x9cpulling pipesxe2x80x9d which are in the original directionally drilled holes.
Another improvement over prior art is the use of the above mentioned molten paraffin applied with conventional jet grouting apparatus. The preferred molten paraffin has a melting point between 120xc2x0 and 180xc2x0 F. and is modified by the addition of a surfactant which allows the molten paraffin to soak into soils which are already water wet or damp, as well as dry soils which have a very low permeability to water. The paraffin may also be replaced by or blended with a low density polyethylene homopolymer.
Previous inventions have addressed forming impermeable caps, vertical barriers and horizontal barriers but the present invention provides a totally integrated solution which results in total isolation of a waste site from the environment in a manner which is continually and passively verifiable. A subsurface xe2x80x9cblockxe2x80x9d or volume of the earth defined by the ground level on its top and by a bottom comprised of a box-shaped or basin-shaped three dimensional mathematical xe2x80x9csurfacexe2x80x9d which surrounds and underlies the block and rises upward to the ground level at the perimeter, forming a complete and continuous basin and top, fully enclosing the volume of earth in an air-tight, and water vapor-tight vault formed in situ around the block.
A liquid grout with viscosity comparable to motor oil, but which is of greater density than the subterranean xe2x80x9cblockxe2x80x9d such that the block will float in the liquid grout, which will subsequently harden into an impermeable barrier material, and where the hardening of this grout is delayed for an extended period of 6 to 60 days while continuing to transmit hydrostatic pressure effectively. The length of set delay and the density and impermeability of this grout is significantly beyond the performance of the previous art.
Directionally drilled holes which traverse the lower surface of the block in roughly parallel paths and which rise to the ground level and level off to a near horizontal attitude at each end. Such holes being formed in a manner which leaves a tubular steel member or xe2x80x9cpipe,xe2x80x9d and one or more non-crossed steel cables, or two pipes and at least two non-crossed cables in each of the holes extending from ground level at one end of the block to ground level at the opposite end of the block. A mechanical earth cutting means consisting of a flexible length of abrasive tensile member such as a steel cable or chain, The catenary section of which is cooled, cleaned and lubricated by a flow of grout from one or more ports in the adjacent pipes which are moved at intervals in synchronous with the net advance of the cutting means, and which itself is joined end to end and reciprocated or circulated in a continuous substantially horizontal loop between the two adjacent holes by a power driven apparatus that maintains tension on the cutting means against the face of the cut. Prior art has not utilized an abrasive cable saw in curving directionally drilled holes and has not anticipated coolant lines advancing through the holes with the cut.
The initial cutting means and periodic replacement cutting means are pulled into the holes by means of the cables initially attached to the pulling pipes. Pipes which have one or more perforations and are used to convey pressurized grout to the arc of the cable saw cut being formed. Movement of such discharge point being accomplished by moving the pipe through the ground or by moving a smaller inner pipe discharging between straddle packers positioned over one or more holes nearest the arc of the cut.
A perimeter excavated trench filled with the dense grout covers each opening into the directionally drilled holes such that the grout may flow by gravity into those into the annulus between the pulling pipe and the hole and into any narrow cut between them formed by the cutting means. Grout may also flow out to relieve pressure. Flow from the grout filled trenches through the annulus to the cut area may be stimulated by a differential elevation of grout in the trench or the grout may flow from the pressurized grout pipe, which traverses the hole and discharges grout at any desired location along the length of the hole. Excess grout will flow up the annulus to the trench or will contribute to increasing the thickness of the barrier.
The cut through the soil along the lower surface of the block, is filled with a layer of the grout such that the overburden weight is supported by the buoyant force of the grout, and such that the thickness of the cut can be increased by adding additional grout to the excavations. The elevation increase of the block may be controlled by changing the elevation of grout in the trench or by changing the grout density. Restraining means such as steel cables or chains, attached between anchorages on the block and anchorages outside the perimeter trench which act to keep the block floating in the center of the excavation from which the block has been lifted, and to limit the elevation increase of any given section of the block.
While the block is floating free on the layer of dense grout, an impermeable sheet, such as high density polyethylene extrusion (HDPE) heat-fusion-seamed together as is known in the art, is attached by chains or other flexible linkage to two or more of the pulling pipes such that the impermeable sheet may be pulled through the layer of liquid grout under the floating block by pulling the pipes from the opposite end until the sheet extends out of the grout filled perimeter trench on all sides. The sheet is preferably heat-fusion-seamed so as to be wide and long enough to underlie the entire block and the outside berm of the perimeter trench. The outermost portions of the sheet are permitted to pucker into undulating folds to compensate for differences in length of the paths under the block. Sites too large to move in one piece may be laid in the grout as unsealed strips with substantial overlap between strips. Separate strips of this material may be equipped with an slidable mechanical interlock, as is known in the art for vertical sheets such as the GSE Gundwall(copyright) Interlock, or Curtain Wall(copyright) made by GSE of Houston, Tex., such that one sheet may be slidably attached to adjacent sheets allowing one sheet to be pulled into place and sealed to its neighbor. A sealing compound may later be injected into this joint from the ends.
An air-tight above ground cap, is then constructed and sealed to the hardened surface of the perimeter trench of, and also preferably to the impermeable sheet. This completes an airtight containment vault over, under and around the block. The top cap may have a layer of impermeable HDPE sheet which is heat-fusion-seam bonded to the bottom liner rising from the perimeter trench so as to form an air-tight seal between the two sheets. The cap is equipped with: air pressure, humidity, sound, and chemical sensors mounted both in the soil under the cap and on its exterior surface such that differential measurements may be performed and recorded on a continual basis in order to evaluate the degree of isolation between the environment inside the structure and the external environment. A standard data logger device records the data from the sensors may be periodically downloaded to a computer which graphically displays the relationship between internal conditions vs external conditions, as a function of time, temperature and rainfall conditions.
A catenary cutting means similar to the cable saw but operating by a reciprocating stroke implemented with standard construction equipment such as trackhoes may also be used to make the cuts between the directionally drilled holes. The apparatus consists of a flexible hollow tube of substantially uniform diameter extending from the surface down through the directionally drilled holes, joined in a catenary arc, through which high pressure fluid is circulated in a continuous loop, and from which at least a portion of this fluid exits the forward face of the tube through one or more holes or xe2x80x9cjetsxe2x80x9d, such that the fluid jet helps erode and wet the soil in the path of the device and allows the fluid to displace substantially all of the soil. The orientation of such fluid jets being cyclically altered to increase the thickness and uniformity of the cut by reciprocating rotation of both ends of the tube an equal increment on each pulling stroke, or by other means substantially in unison such that all soil in the path of the tube can be impacted by one or more fixed jets. The surface of the catenary tube is abrasive and mechanically cuts the soil in its path as well as eroding it with fluid jets. An additional abrasive cable may be pulled into the cut by means of the color-coded, non-crossing cables on the pulling pipe. This cable can bypass the tube and perform an abrasive cutting job and then be withdrawn from either end. The entire cutting tube could also be circulated out of the ground and temporarily replaced by an abrasive cable or chain. If the tube is damaged it can also be replaced in the same manner. This is a major improvement over jet cutting methods which have no recourse when they strike a hard object or if the jets plug. If the jetting tube has substantial enlargements along its length or at the slurry discharge points then it can not be circulated out of the hole if a problem should develop. This ability to recover from a structural failure, jet plugging, or a hard obstruction is critical to commercial use of the process.
The grout material may be either a slow setting dense material capable of buoyantly supporting the overburden or may be a fast set or thermoplastic set material which sets before a large unsupported span exists. A low water, cementitious, latex polymer modified grout with iron oxide additives and a long term set retarder is preferred for buoyant barriers. A molten a grouting material made from paraffin wax or polyethylene homopolymer and surfactant admixtures which enable it to mix with damp or wet soils and permeate farther into water impermeable soils is preferred for the non-buoyant process. Circulation of molten grout through the pulling pipes and the catenary tube can keep the material from setting during a work delay or even overnight. Paraffin supply lines from relatively hot and relatively cool but molten paraffin may be blended by a valve to rapidly adjust the temperature of the material with changing ground conditions. Blends of paraffin and polyethylene may also be used. A cap liner made of a similar polyethylene or paraffin mixture may be used in the top cap and heat fusion bonded to the bottom barrier to create a completely air tight seal of similar material. This cap material may be sprayed onto the surface of the cap as a liquid material and cured in place or it may be a pre-fabricated sheet.
The above mentioned grouts have desirable properties for block encapsulation of buried low level radioactive waste. The molten wax and surfactant blends offer superior permeation into non-homogenous trash as well as good bonding and encapsulation of organic sludges. They offer a desirable matrix to stabilize the waste while it remains in the ground and also prevent airborne dust release during future retrieval. Since they are fully combustible they add no volume to the final waste matrix of a vitrification melter process.